Freeing seized piston: Hope Mono Mini

[moonlite]

Futzing about with the Hope Minis I plan to use on the Kula Primo build I'm working on, I have entered the rabbithole of hydraulic disc maintenance.
Opening up the Minis removed some of the reservations I'd had about working on these brakes, and emboldened me to take a look at a Mono Mini that had languished in the spares box ever since my LBS replaced it due to a stuck piston.

The bike had been stored in a roofed but open-sided parking area in Singapore where the heat and humidity had done its worst. Anyway, I have now stripped down the caliper and tried everything recommended by experts on the web: applying silicone grease, silicone spray, air pressure through the bleed nipple, etc. I have also heated the outside of the caliper to see if metal expansion could shift it. Nothing.

I have a spare piston ready to go as an acknowledgement that my efforts will probably damage the stuck one. But it's still there, seized in the retracted position where I can't get any purchase on it. The piston material is really tough, too.

I really don't want to damage the caliper itself, so what to do?

Any advice gratefully received.

 

[moonlite]

Some googling on the properties of phenolic resin, which the piston is made of, suggests that it may be soluble in acetone. Any thoughts on this?

 

[mdvineng]

Acetone will work but slowly. Quicker option is to accurately mount the caliper in a machine vice and use slot end milling cutters to remove the inner section of the piston. Remove enough until you can collapse the remaining wall section that's left with a pick. Best done on a milling machine or a well clamped drill press, never use a handheld drill!

 

[clubby]

Can see gouging in the piston and calliper body, presumably from attempts to remove it. Unless you have the proper tools to mill out the piston, I’d just replace. Spare callipers on eBay for £25. On the other hand just go for it in the knowledge it’s an easy swap if you screw it up. Personally for £25 I’d just buy a calliper.

 

[moonlite]

Acetone will work but slowly. Quicker option is to accurately mount the caliper in a machine vice and use slot end milling cutters to remove the inner section of the piston. Remove enough until you can collapse the remaining wall section that's left with a pick. Best done on a milling machine or a well clamped drill press, never use a handheld drill!

Good advice -- would that I could! Unfortunately I don't have access to that kind of machinery. Perhaps I could drill a couple of holes through the piston and use circlip pliers or similar to twist/lift it out.

 

[moonlite]

Can see gouging in the piston and calliper body, presumably from attempts to remove it. Unless you have the proper tools to mill out the piston, I’d just replace. Spare callipers on eBay for £25. On the other hand just go for it in the knowledge it’s an easy swap if you screw it up. Personally for £25 I’d just buy a calliper.

No gouging -- I think what you're seeing is the remains of the silicone grease in there.

The problem with just buying a used caliper on eBay is that the current exchange rate is 52p to the Aussie dollar, so a £25 caliper would be AU$ 50+ -- and that's before postage, which if done via eBay's shipping programme would probably be another £20 or so. Not really feasible.

And of course, there would still be a nice caliper with a stuck piston in it, bothering me.

 

[mdvineng]

Good advice -- would that I could! Unfortunately I don't have access to that kind of machinery. Perhaps I could drill a couple of holes through the piston and use circlip pliers or similar to twist/lift it out.

Yes, two small holes diametrically apart might work.

 

[dyna-ti]

Good advice -- would that I could! Unfortunately I don't have access to that kind of machinery. Perhaps I could drill a couple of holes through the piston and use circlip pliers or similar to twist/lift it out.

The piston is solid so mind you dont drill right through the caliper too

 

[moonlite]

The piston is solid so mind you dont drill right through the caliper too

Yep -- there's always that risk

 

[novocaine]

rebuild it. clamp the other piston. bleed it up and use hydraulic pressure to push it out. it's unlikely to fail to move this way.